1. Field of the Invention
The present invention relates to a catadioptric optical system and a projection exposure apparatus equipped with the catadioptric optical system suitable when manufacturing in a photolithography process, for example, a semiconductor device or a liquid crystal display device. In particular, the invention relates to a catadioptric optical system suitable for a scanning type projection exposure apparatus.
2. Related Background Art
In a photolithography process for manufacturing semiconductor devices and the like, there is used a projection exposure apparatus by which a pattern image formed on a photomask or reticle (collectively referred to as “reticle” hereinafter) is projected and exposed onto a wafer, a glass plate, etc. coated with a photoresist or the like via a projection optical system. As the integration of the semiconductor devices and the like is improved, there has been a demand for a higher resolution of the projection optical system used in the projection exposure apparatus. In order to satisfy such a demand, there have been occurred necessities for shortening the wavelength of illumination light and increasing the numerical aperture (hereinafter referred to as “NA”) of the projection optical system. In particular, regarding the exposure wavelength, replacing g-line (λ=436 nm), i-line (λ=356 nm) and, further, KrF excimer laser light λ=248 nm) are currently used. In the future, ArF excimer laser light (λ=193 nm) and F2 laser light (λ=157 nm) will probably be used.
However, as the wavelength of the illumination light becomes shorter, a fewer kinds of glass materials can be practically used due to light absorption. As a result, when the projection optical system is constructed by a refraction system alone, that is, only by optical elements not including a reflecting mirror with refractive power (a concave or convex mirror), chromatic aberration cannot be corrected. Additionally, because the optical performance required of the projection optical system is extremely high, various kinds of aberrations should preferably be corrected to a level of almost no aberration. Eighteen or more lens elements are required for correcting various aberrations to a desired optical performance by a refraction type projection optical system constituted of lens elements (see, for example, Japanese Unexamined Patent Publication Hei No. 5-173065), and it is difficult to suppress light absorption and avoid manufacturing costs' increase. Moreover, when extreme ultraviolet light with a wavelength of 200 nm or less is used, the optical performance may be affected by, for example, light absorption in glass material and on an anti-reflection film on the lens surface.
Further, although the oscillation bandwidth of laser light sources with an oscillation wavelength of 200 nm or less has been considerably narrowed, the bandwidth has still a certain wavelength width. Thus, to project and expose a pattern maintaining good contrast, correction of chromatic aberration of the order of pm (pico meter) is required. The optical system disclosed in the above-mentioned Japanese Unexamined Patent Publication Hei No. 5-173065 is a refraction type lens system made from a single kind of glass material, and its chromatic aberration is too large to be used with a light source having a wavelength width.
On the other hand, a reflection type optical system utilizing power (refractive power) of a concave mirror and the like does not effect chromatic aberration and, with respect to Petzval sum, creates a contribution with an opposite sign to a lens element. As a result, a so-called catadioptric optical system (hereinafter referred to as “catadioptric optical system”), which combines a catoptric optical system and a dioptric optical system together, can correct chromatic aberration as well as other various aberrations to a level of almost no aberration without increasing the number of lenses. Thus, a catadioptric optical system is an optical system having at least one lens element and at least one reflecting mirror with refractive power.
However, when a concave mirror is incorporated on the optical axis of a projection optical system of a projection exposure apparatus, light from the reticle side incident on the concave mirror is reflected toward the reticle. Addressing this problem, techniques to separate the optical path of light incident on a concave mirror from the optical path of light reflected by the concave mirror and also to direct the reflected light from the concave mirror to the wafer direction, i.e., various techniques to implement a projection optical system by a catadioptric optical system, have been extensively proposed.
However, when using a beam splitter as is used in the optical system disclosed in Japanese Unexamined Patent Publication Hei No. 5-281469, it is difficult to secure large-sized glass material for manufacturing the optical system. In addition, in the case of another proposed optical system, an optical path folding mirror (folding mirror) or a beam splitter is required and a plurality of lens barrels are required for manufacturing the optical system, resulting in such problems as difficulties in manufacture or in adjusting optical elements. A light beam impinges obliquely onto a plane reflecting mirror (folding mirror) for changing the optical path direction incorporated in a catadioptric optical system as necessary. Accordingly, extremely high surface accuracy of the mirror is required, resulting thus in the difficulty of the manufacture of the mirror. Further, the mirror is easily affected by vibration.
Meanwhile, when an optical path separating method disclosed in U.S. Pat. No. 5,717,518 is used, optical elements constituting a optical system can all be disposed along a single optical axis. As a result, the optical system can be manufactured with high accuracy following an optical element adjustment method conventionally used in the projection optical system manufacture. However, the system requires a central light-shielding portion to shield light beam propagating along the optical axis, resulting in the contrast deterioration of a pattern of a certain frequency.
Additionally, because it is difficult to provide an anti-reflection film with sufficient optical performance in the extreme ultraviolet wavelength region, it is also required that the number of optical elements constituting an optical system be reduced as much as possible.
As can be seen from the above, it is preferable that, to expose a pattern having a linewidth of 0.18 μm or less, an optical system in which a good chromatic aberration correction capability is realized even when using a light source with a wavelength of 200 nm or less such as ArF or F2 laser, no central light-shielding is used, a high numerical aperture of NA 0.6 or more can be secured, and the number of refractive and reflecting components is reduced as much as possible be provided